1. Field of the Invention
The present invention relates to an apparatus for steering a belt including a steering roller supporting the belt, a rotatable element and a transmission. The steering roller is rotatable about a first axis and pivotable about a second axis, the second axis being substantial perpendicular to the first axis. The rotatable element is rotatable about a third axis, the third axis being parallel to the first axis. The transmission is arranged for converting a rotational motion of the rotatable element about the third axis into a pivoting motion of the steering roller about the second axis. The rotatable element is rotatable by a frictional force between a side portion of the belt and the rotatable element.
2. Description of Background Art
An apparatus of this kind is known from EP 0 458 260. In this patent, a belt driving system having at least one roller for adjusting creep within a plurality of rollers is disclosed. The belt driving system disclosed in EP 0 458 260 includes: a creep detecting means provided at one end of the creep adjusting roller which is rotated by torque of a flat belt in contact with the creep detecting means; a biasing means for biasing the flat belt towards the creep detecting means; and a roller-end displacing means for converting the torque of the creep detecting means into a displacement of the end of the creep adjusting roller towards a predetermined direction so that the flat belt is moved to the direction contrary to the creep caused by the biasing means. When the flat belt creeps, the creep contrary to the original creep is caused by the roller-end displacing means, and thus the original creep is compensated. Consequently, stable running of the flat belt is obtained.
Although the apparatus known from the cited reference enables steering of a belt to its desired path, it has the disadvantage that relatively large steering forces and hence large frictional forces between the rotatable element and the side portion of the running belt are required to operate the steering mechanism. The construction of the steering mechanism disclosed in EP 0 458 260 is such that a relatively large contact area between the rotatable element, which is the creep detection means in the cited reference, and a surface area of the belt is required to provide for the steering force to obtain stable running of the belt. A large contact area between the rotatable element and the belt increases the risk of damaging the belt during operation.
Practice has shown that, unlike for relatively long belts, the running path of relatively short belts is extremely sensitive towards torsion of the frame that holds the rollers that support the belt. Long belts can easily be steered by passive steering flanges (fixed non-rotatable elements that restrict lateral displacement of the belt). Therefore, short belts cannot be steered in this way. Short belts run off the rollers and get damaged beyond repair.